A propeller drive of the above-mentioned type is constructed to meet the demands of the market for much faster boats with much larger and more powerful motors. In order to maintain or increase the operating life of the propeller drive with a much greater effective output, a need arises for a gearbox of correspondingly larger size in relation to a given propeller diameter. In order to avoid cavitation problems at the transition from the gearbox to the propeller hub, it is traditional to strive to dimension the diameter of the propeller hub in such a way that the propeller hub is connected to the gearbox in a “straight” transition, thus without a change in dimension.
An increase in the diameter of the propeller hub can, however, for practical reasons, not always be accompanied by a corresponding increase in the diameter of the propeller since it is known from previous propeller experiments that the degree of efficiency of the propeller drops when the diameter of the propeller hub exceeds about 25% of the propeller diameter. The problem thus arises that the gearbox must be dimensioned so large, for reasons related to power or stability to stress, that the diameter of the propeller hub, in the case of a straight transition between the gearbox and the propeller hub, must exceed the diameter of the propeller by significantly more than 25%.
The problem has therefore been considered to be unsolvable in general, since a conventional straight or slightly curved transition cone has turned out to result in undesirable cavitation around the propeller hub because dissolving takes place already at the first, front end of the transition cone, which is located upstream. The cavitation around the propeller hub also entails a big problem with cavitation erosion of the propeller blades against the root parts adjacent to the hub, loss of efficiency, with the consequence of unfavorable flow behavior in the cavitation zone around the root parts, and pressure impulses at the entrance end of the hub.
As a consequence of the fact that problems are encountered with an enlarged gearbox in comparison with the diameter of the propeller both if a larger hub diameter is selected (leading to a drop in the degree of efficiency of the propeller drops) and if a thin propeller hub is retained in conjunction with a conventional transition cone (leading to cavitation erosion and loss of efficiency), a convention has developed among designers that the gearbox should generally not be dimensioned larger than 25% of the propeller diameter. As mentioned in the introduction, however, in modern high-power motor-drive combinations there is no need to over-dimension the gearbox of the propeller drive in relation to a given propeller diameter in order to maintain or increase the operating life of the propeller drive with this high power output.